This invention relates generally to electrodialysis of a salt solution for generating acid and base solutions and, more particularly, to an improved method and four channel bipolar membrane stack for producing low chloride sodium hydroxide.
It is well known in the art that acid and base solutions may be generated from a salt solution by the electrodialysis water splitting process. In general, the electrodialysis process consists of a stack containing a plurality of cation-selective membranes, bipolar membranes, and anion-selective membranes positioned between a pair of electrodes. The stack itself comprises an assembly of a plurality of unit cells, each unit cell comprising the said membranes arranged to provide a plurality of parallel flow paths or channels therebetween. When a direct electrical current is applied to the bipolar membrane, water is split into hydroxyl ions and hydrogen ions which migrate or travel to the anode and cathode respectively. At the same time, the cation-selective membrane passes cations traveling toward the cathode while blocking anions, and the anion-selective membrane passes anions traveling toward the anion while blocking cations. If a salt solution such as sodium chloride is directed through the channel between the cation and anion-selective membranes, the concentration of that solution is depleted as hydrochloric acid and sodium hydroxide are formed in adjacent acid and base containing channels. With suitable controls, pumps and associated collecting vessels, the acid and base may be withdrawn for subsequent use and the concentration of the salt solution replenished as required.
An electrodialysis process of the type indicated is fully described in the commonly assigned U.S. Pat. No. 4,880,513, and the teachings of that patent are incorporated herein by reference. The method disclosed in said patent is a fill and draw or batch method wherein various controls, such as pH and liquid level switches are utilized to periodically withdraw the desired acid and base and replenish the raw material salt solution. That process also utilizes a common bipolar membrane stack consisting of three-channel unit cells comprising a bipolar membrane having an anion-selective membrane on one side thereof and a cation-selective membrane on the opposite side thereof.
It has been found that the production of sodium hydroxide from a plentiful salt like sodium chloride in a bipolar membrane stack produces sodium hydroxide that may be contaminated with small amounts of chloride ions, apparently resulting from leakage of chloride ions through the bipolar membrane. For certain applications, such as regenerating anion-exchange resins for use in nuclear power stations, low chloride or substantially pure sodium hydroxide is required. The chloride level in sodium hydroxide produced by a bipolar membrane stack can be reduced by improvements in the anion side of the bipolar membrane, but usually only at the expense of higher membrane resistance which requires more power and cooling water to operate the stack.
The chloride level in sodium hydroxide produced by a bipolar membrane stack can also be lowered by operation in a fill and draw batch method of the type described. That method utilizes the fact that the amount of chloride contamination increases as the concentration of the sodium hydroxide is increased. However, that process must be operated as a batch process and there are upper limits to the sodium hydroxide concentration for a given chloride contamination.
There thus exists a need for a low-cost method and apparatus for producing sodium hydroxide with reduced chloride contamination by electrodialysis of a salt in a bipolar membrane stack.